WASHINGTON -- Federal agents seized truckloads of stolen military and civilian weapons yesterday in an expanding investigation into the theft and sale of machine guns, grenades and plastic explosives that centers on the Marine base at Camp Lejeune, N.C.

Meanwhile, more Marines with ties to Lejeune are likely to be arrested beyond the six taken into custody Thursday, said a senior investigator, who spoke on the condition that he not be named.

Throughout the day yesterday and Thursday, agents from the FBI and the Bureau of Alcohol, Tobacco and Firearms gathered weapons from at least five homes and commercial storage sites throughout North Carolina and in a town in western Virginia.

The investigation -- "Operation Longfuse" -- was triggered last year when a Marine officer at Lejeune noticed missing ordnance and informed federal investigators, who employed undercover agents in their probe, the senior investigator said.

Investigators said some of the recovered ordnance was stolen from Lejeune, the coastal Marine base, but weapons from other services may also have been seized.

"Agents believe there are other items other than Marine Corps inventory," said one source, adding that "at least two truckloads" of weapons have been recovered.

Defense Secretary William S. Cohen has directed all the services to review security and accountability for small arms and ammunition and to report back within 30 days.

"It is imperative that we be confident we have procedures in place to ensure that military arms and munitions are tightly controlled and safeguarded," Cohen said.

Six Marines and eight civilians have been arrested so far in the federal investigation, which has included agents from the Naval Criminal Investigative Service in addition to those of the FBI and Bureau of Alcohol, Tobacco and Firearms.

One investigator, speaking on condition of anonymity, said there is "a possible link" between the civilian suspects and militia groups, based on information assembled in the course of the probe. "Some of them have ties -- potentially -- to militia groups," said the investigator.

No evidence, however, has surfaced to link the Marines to any militia groups, the investigator said. He added that most of the Marines and some of the civilians arrested are cooperating with investigators.

One of the Marines, Capt. Thomas A. Crawford, a 38-year-old decorated Desert Storm veteran with 21 years of service, was arrested Thursday in Millis, Mass., about 30 miles southwest of ,, Boston, where investigators seized at least 10 crates with military markings.

Crawford and four other Marines are being held at the Lejeune brig pending charges, said Capt. Ricardo T. Player, a spokesman at the Marine base. The sixth Marine was released from custody because of information he provided to investigators, but is still being monitored at the base, said Player. Crawford is the only officer to have been arrested.

Sgt. Jonathan Cress, another Lejeune spokesman, said investigators have determined that hand grenades, plastic explosives and detonation cord were stolen from Lejeune, though the haul to date has also included machine guns, grenade launchers, Claymore mines and a mortar.

Federal agents are still trying to determine where the other weapons came from, noting that some are military and others are civilian in origin, said FBI Special Agent JoAnne Morley in Charlotte, N.C.

Five houses and storage areas were raided by agents in North Carolina, along with a sixth location in Abingdon, Va., an Appalachian town just above the North Carolina border, she said.

Asked about a possible militia link, Morley said:

"We can't say that at this point in time. Our investigation is continuing." She noted that all the civilians were charged as individuals. "We're not charging them as a ring or a group."

Cress, the Lejeune spokesman, said the arrested Marines were involved in disposing of explosive ordnance.

While they all knew each other, there is no evidence they were organized, he said. "There was no ring or group," Cress said. "They may have known that the others were involved, but they never acted together."

Some of the seven civilians arrested, all from North Carolina, were identified as weapons dealers. They have already been charged with a variety of federal weapons violations.

The Marines were able to obtain the grenades, plastic explosive and detonation cord because they had access to them through their work in ordnance disposal, said Cress, and were somehow able to foil security procedures, which include the requirement for multiple signatures to withdraw each type of explosive device.

"They had access to this type of ordnance and took advantage of the rules," said Cress, who could not offer specifics on those procedures. "Most Marines can't draw out a hand grenade anytime they want to."

Among the weapons undercover agents were able to purchase from those arrested were 150 pounds of C-4 explosive, a 50-caliber machine gun, 40 smaller machine guns, a 60 mm mortar, 100 grenades and an unspecified number of Claymore mines.

They were purchased either from the Marines or the civilians, who previously bought them from the Marines, said an investigator.